This invention relates to a sensing pad assembly for monitoring acoustic activity and/or motion of an object supported on the pad. The invention is more particularly concerned with a sensing pad assembly that utilizes an improved variable coupler fiberoptic sensor as a pressure transducing element. The sensing pad is suitable for use in a variety of monitoring applications and is especially useful in systems for continuously monitoring medical patients, or more generally, human subjects.
It is commonplace in medical practice to continuously monitor a patient""s vital signs, such as heart rate and respiration rate, for changes that may indicate deterioration of the patient""s condition. Continuous monitoring systems typically require the attachment of electrical and/or physical sensors to the patient""s body using adhesive or straps. Such sensors are generally uncomfortable to the patient and often limit patient activity due to the presence of the sensors, straps, and associated sensor leads. Moreover, such monitoring systems are subject to false outputs due to unreliability of skin contact with patient movement.
An alternative form of monitoring system has been proposed in which the patient is supported on a sensing pad having an associated pressure transducer that does not contact the patient. Acoustic activity and motion of the patient generate pressure fluctuations in the material of the pad. These fluctuations, which vary in accordance with the parameter or parameters being monitored, propagate through the pad material to the transducer. The transducer then converts them to electrical signals for processing by a monitoring circuit.
U.S. Pat. No. 5,684,460 to Scanlon (hereinafter referred to as xe2x80x9cthe ""460 patentxe2x80x9d) discloses one example of a monitoring system as just described. FIG. 1 illustrates this system in simplified block diagram form. The system includes a fluid-filled sensing pad 1 adapted to support a patient and to transmit pressure fluctuations due to the patient""s acoustic activity or motion to a pressure transducer 2 that converts the pressure fluctuations to an electrical output. The pressure transducer is coupled to the internal fluid medium 3 of the pad via a hose 4. A monitoring circuit 5 monitors the output from the transducer and provides outputs to activate a patient stimulator 6 and an alarm 7 upon the occurrence of predetermined conditions, such as when the transducer output corresponds to no sound and/or movement (below a predetermined threshold) or indicates abnormal activity of the patient.
The sensing pad 1 may be in the form of a fluid-filled mattress or configured for use in some other suitable support such as a vehicle seat or a stroller. Proposed applications of the system for monitoring human subjects include the monitoring of infants at risk for sudden infant death syndrome (SIDS), controlling sleep apnea and snoring, and sensing the onset of sleep for drivers of motor vehicles. Other proposed applications include monitoring machinery for noises and vibrations indicative of atypical operation.
The ""460 patent mentions several classes of sensors as being suitable for use as the pressure transducer. Examples include electrical, mechanical, piezoelectric, and fiberoptic sensors.
One type of fiberoptic sensor not explicitly mentioned in the ""460 patent, but known to have performance characteristics that are especially well suited for patient monitoring and a variety of other applications, is the variable coupler fiberoptic sensor.
Variable coupler fiberoptic sensors conventionally employ so-called biconical fused tapered couplers manufactured by a draw and fuse process in which a plurality of optical fibers are stretched (drawn) and fused together at high temperature. The plastic sheathing is first removed from each of the fibers to expose the portions for forming the fusion region. These portions are juxtaposed, usually intertwisted one to several twists, and then stretched while being maintained above their softening temperature in an electric furnace or the like. As the exposed portions of the fibers are stretched, they fuse together to form a narrowed waist regionxe2x80x94the fusion regionxe2x80x94that is capable of coupling light between the fibers. During the stretching process, light is injected into an input end of one of the fibers and monitored at the output ends of each of the fibers to determine the coupling ratio. The coupling ratio changes with the length of the waist region, and the fibers are stretched until the desired coupling ratio is achieved, typically by a stretching amount at which the respective fiber light outputs are equal. The coupler is drawn to such an extent that, in the waist region, the core of each fiber is effectively lost and the cladding may reach a diameter near that of the former core. The cladding becomes a new xe2x80x9ccore,xe2x80x9d and the evanescent field of the propagating light is forced outside this new core, where it envelops both fibers simultaneously and produces the energy exchange between the fibers. A detailed description and analysis of the biconical fused tapered coupler has been given by J. Bures et al. in an article entitled xe2x80x9cAnalyse d""un coupleur Bidirectional a Fibres Optiques Monomodes Fusionnesxe2x80x9d, Applied Optics (Journal of the Optical Society of America), Vol. 22, No. 12, Jun. 15, 1983, pp. 1918-1922.
Biconical fused tapered couplers have the advantageous property that the output ratio can be changed by bending the fusion region. Because the output ratio changes in accordance with the amount of bending, sensors employing such couplers can be used in virtually any sensing application involving motion that can be coupled to the fusion region. For example, U.S. Pat. No. 5,074,309 to Gerdt discloses the use of such sensors for monitoring cardiovascular sounds including both audible and sub-audible sounds from the heart, pulse, and circulatory system of a patient. Other applications of variable coupler fiberoptic sensors can be found in U.S. Pat. No. 4,634,858 to Gerdt et al. (disclosing application to accelerometers), U.S. Pat. No. 5,671,191 to Gerdt (disclosing application to hydrophones), and elsewhere in the art.
Conventional variable coupler fiberoptic sensors have relied upon designs in which the fiberoptic coupler is pulled straight, secured under tension to a plastic support member and, in the resulting pre-tensioned linear (straight) form, encapsulated in an elastomeric material such as silicone rubber. The encapsulant forms a sensing membrane that can be deflected by external forces to cause bending of the coupler in the fusion region. The bending of the fusion region results in measurable changes in the output ratio of the coupler. The displacement of the membrane can be made sensitive to as little as one micron of movement with a range of several millimeters.
FIG. 2 of the accompanying drawings illustrates the basic principles of a sensing apparatus including a variable coupler fiberoptic sensor 10 as described above. In the form shown, the sensor 10 includes a 2xc3x972 biconical fused tapered coupler 11 produced by drawing and fusing two optical fibers to form the waist or fusion region 13. Portions of the original fibers merging into one end of the fusion region become input fibers 12 of the sensor, whereas portions of the original fibers emerging from the opposite end of the fusion region become output fibers 14 of the sensor. Reference numbers 18 denote the optical fiber cores. The fusion region 13 is encapsulated in an elastomeric medium 15, which constitutes the sensing membrane. The support member is not shown in FIG. 1.
In practice, one of the input fibers 12 is illuminated by a source of optical energy 16, which may be an LED or a semiconductor laser, for example. The optical energy is divided by the coupler 11 and coupled to output fibers 14 in a ratio that changes in accordance with the amount of bending of the fusion region as a result of external force exerted on the sensing membrane. The changes in the division of optical energy between output fibers 14 may be measured by two photodetectors 17 which provide electrical inputs to a differential amplifier 19. Thus, the output signal of differential amplifier 19 is representative of the force exerted upon medium 15. It will be appreciated that if only one of the input fibers 12 is used to introduce light into the sensor, the other input fiber may be cut short. Alternatively, it may be retained as a backup in the event of a failure of the primary input fiber. It should be noted that, for simplicity, the coupler 11 is shown without the aforementioned fiber twisting in the fusion region. Such twisting is ordinarily preferred, however, to reduce lead sensitivity, which refers to changing of the output light division in response to movement of the input fiber(s).
As compared with other types of fiberoptic sensors, variable coupler sensors offer a uniquely advantageous combination of low cost, relatively simple construction, high performance (e.g., high sensitivity and wide dynamic range), and versatility of application. Other known fiberoptic sensors have used such principles as microbending loss, light phase interference, and polarization rotation by means of birefringence. Fiberoptic micro-bending sensors are designed to sense pressure by excluding light from the fiber in proportion to the changes in pressure. The output light intensity decreases with increases in measured pressure, as pressure is transduced into light loss. Because the measurement accuracy is reduced at lower light levels, the dynamic range of such sensors is severely limited. Interferometric fiberoptic sensors measure changes in pressure by applying pressure to an optical fiber to change its index of refraction. This results in a phase delay that is measured by utilizing a Mach-Zehnder or Michaelson interferometer configuration. These sensors are extremely expensive and require sophisticated modulation techniques that render them unsuitable for many applications. Polarization varying fiberoptic sensors alter the polarization state of a polarized optical signal in accordance with a change in temperature or pressure. Such polarized light sensors require special optical fiber and expensive polarizing beam splitters.
Despite their advantages, conventional variable coupler fiberoptic sensors have been subject to certain limitations inherent in their pre-tensioned linear (straight) coupler design. The conventional design imposes, among other things, significant geometrical limitations. In particular, the size of the sensor must be sufficient to accommodate the fiberoptic leads at both ends of the sensor. The fiberoptic lead arrangement also requires the presence of a clear space around both ends of the sensor in use. Another limitation results from the fact that any displacement of the fusion region necessarily places it under increased tension. At some point of displacement, the tension in the fusion region will become excessive, causing the fusion region to crack or break, with resulting failure of the coupler.
The present invention provides a sensing pad assembly that uses an improved variable coupler fiberoptic sensor designed to overcome one or more disadvantages of the conventional pre-tensioned linear sensor design. More particularly, the sensor used in the present invention may have an improved design that permits deflection of the coupler fusion region without accompanying tension. The coupler fusion region is preferably arranged substantially in a U-shape, but may more generally be configured as disclosed in co-pending U.S. application Ser. No. 09/316,143 filed May 21, 1999, which is incorporated herein by reference. With a substantially U-shaped configuration, it becomes possible to locate the fiberoptic leads adjacent to each other rather than at opposite ends of the sensor, thus avoiding the earlier discussed geometrical limitations inherent in the conventional pre-tensioned linear coupler design. The pad may be of any desired configuration so long as it can transmit pressure fluctuations to be monitored to the variable coupler fiberoptic sensor. For example, the pad may be in the form of a mattress, a sheet-like member to be placed upon or beneath a mattress, a seat cushion, or a sheet-like member incorporated in a sheet cushion.
In one preferred embodiment of the invention, the sensor is disposed within the internal material of the sensing pad. In another preferred embodiment, the sensor is secured to an outer surface of the pad with its sensing area (containing at least part of the fusion region) coupled to the internal medium of the pad via a hole formed in the pad outer surface. Generally speaking, however, the sensor may be arranged in any manner that couples the sensing area so as to receive pressure fluctuations propagated by the material of the pad.
In summary, according to one of its principal aspects, the present invention provides a sensing pad assembly that comprises a pad member having a surface configured to support an object to be monitored thereon, the pad member being capable of transmitting pressure fluctuations due to acoustic activity or motion of the supported object. The assembly further comprises a variable coupler fiberoptic sensor having a fused-fiber coupling region, with at least a portion of the coupling region being disposed in a sensing area of the sensor and configured such that it can be deflected to change an output of the sensor without the coupling region being put under tension. The sensing area is disposed such that the pressure fluctuations are transmitted thereto to deflect the aforementioned portion of the coupling region and thereby change the output of the sensor in accordance with the pressure fluctuations.
According to another of its principal aspects, the invention provides a sensing pad assembly that comprises a pad member as characterized above, and a variable coupler fiberoptic sensor having a substantially U-shaped, fused-fiber coupling region disposed in a sensing area of the sensor such that the coupling region can be deflected to change the output of the sensor. The sensing area is disposed such that the pressure fluctuations are transmitted thereto to deflect the coupling region and thereby change the output of the sensor in accordance with the pressure fluctuations.